It tries to give (admittedly fairly naive) long-term mining return forecasts using current data pulled from the bitcoin network and Mt Gox. It assumes mining difficulty is correlated with price, see the thread on here about that... it was shown to be historically correct as far back as Nov '10 I believe.

Well, anyway, there you have it. Suppose you buy right now a mining rig for $900 that gives you 900MH/s and that you withdraw 10% (that's an estimate taking in account that price rises and then you have to withdraw a lower % to pay electricity bills) of the generated bitcoins. In the end you'll have 89.6 BTC. If you had just bought $900 worth of bitcoins you would have 34.6 BTC.

And if price doesn't rise for a time, then difficulty won't (well it will because people get more efficient but not so fast) either and you'll have more bitcoins. You have to stop thinking in dollars and start thinking in bitcoins!

That's what I wanted to explain to people when they say that you earn more money by just investing in bitcoins. That's not true, with mining rigs you are generating bitcoins and you only have to withdraw enough to pay electricity bills.

I guess with such a tool you can also calculate a USD per MH/s rate under which it would be more profitable to buy, right?

Mining difficulty might be correlated with price - BUT there are and have been serious shifts in there as well!

I really should create a few graphs on my own... I just don't find more convenient MtGox data (idela would be daily weighted averages or even block/2016 block weighted averages!) so I'd have to mess with huge tables in Excel just to prove a point that currently noone seems to realize anyways.

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I guess with such a tool you can also calculate a USD per MH/s rate under which it would be more profitable to buy, right?

Mining difficulty might be correlated with price - BUT there are and have been serious shifts in there as well!

I really should create a few graphs on my own... I just don't find more convenient MtGox data (idela would be daily weighted averages or even block/2016 block weighted averages!) so I'd have to mess with huge tables in Excel just to prove a point that currently noone seems to realize anyways.

If you want to buy rigs with bitcoins, buy them when $/btc are high. Difficulty can and will rise with price, but you'll end with more bitcoins than you had before.

I guess with such a tool you can also calculate a USD per MH/s rate under which it would be more profitable to buy, right?

It's up at the top as "PH ratio" or price-hashrate ratio.

I would think that buying a rig vs buying BTC directly have entirely different risks... the former to me has always seemed less risky because in the end you can always sell the rig, that is, there is much less chance of the computer hardware market suddenly crashing.